Methods and apparatus for phased array imaging

ABSTRACT

A method of imaging a scene includes generating a temporally varying optical intensity pattern from at least one continuous wave (CW) light beam. The method also includes illuminating at least one portion of the scene with the temporally varying optical intensity pattern so as to cause a photon to scatter or reflect off the at least one portion of the scene. The photon reflected or scatted from the at least one portion of the scene is detected using a single-photon detector. Based on the temporally varying optical intensity pattern and a time of flight of the photon detected, a distance between the single-photon detector and the at least one portion of the scene is estimated.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/939,213, now U.S. Pat. No. 10,073,177, filed Nov. 12, 2015, entitled“METHODS AND APPARATUS FOR PHASED ARRAY IMAGING,” which claims priority,under 35 U.S.C. 119(e), to U.S. Application No. 62/079,729, filed Nov.14, 2014, entitled “OPTICAL PHASED ARRAY LADAR.” Each of theseapplications is hereby incorporated herein by reference in its entirety.

GOVERNMENT SUPPORT

This invention was made with Government support under Contract No.FA8721-05-C-0002 awarded by the U.S. Air Force. The Government hascertain rights in the invention.

BACKGROUND

There are generally two types of remote sensing technologies: passivesensing and active sensing. In passive sensing, images or otherrepresentations of a target are created by detecting radiation that isgenerated by an external source, such as the sun. In contrast, activesensing technologies not only detect radiation reflected or scattered bya target but also generate the radiation to illuminate the target forsubsequent detection.

Light Detection and Ranging (LIDAR, also known as LADAR) is an activesensing technique that involves emitting light (e.g., pulses from alaser) and detecting the reflected or scattered light. LIDAR typicallymeasures the time-of-flight (i.e., the time it takes for the pulse totravel from the transmitter to the target, be reflected, and travel backto the sensor), which can be used to derive ranges (or distances) to thetarget which reflects or scatters the light. In this manner, LIDAR isanalogous to radar (radio detecting and ranging), except that LIDAR isbased on optical waves instead of radio waves.

LIDAR can be airborne or ground-based. Airborne LIDAR typically collectsdata from airplanes looking down and covering large areas of the ground.LIDAR can also be conducted from ground-based stationary and mobileplatforms. Ground-based LIDAR techniques can be beneficial in producinghigh accuracies and point densities, thus permitting the development ofprecise, realistic, three-dimensional representations of scenes such asrailroads, roadways, bridges, buildings, breakwaters, or shorelinestructures.

SUMMARY

Embodiments of the present invention include apparatus, systems, andmethods of imaging a scene using sparse apertures. In one example, amethod of imaging a scene includes generating a temporally varyingoptical intensity pattern from at least one continuous wave (CW) lightbeam. The method also includes illuminating at least one portion of thescene with the temporally varying optical intensity pattern so as tocause a photon to scatter or reflect off the at least one portion of thescene. The photon reflected or scatted from the at least one portion ofthe scene is detected using a single-photon detector. Based on thetemporally varying optical intensity pattern and a time of flight of thephoton detected, a distance between the single-photon detector and theat least one portion of the scene is estimated.

In another example, an apparatus for imaging a scene includes a phasedarray, at least one single-photon detector, and a processor. The phasedarray illuminates a portion of the scene with a time-varying opticalintensity pattern generated from at least one continuous wave (CW) lightbeam so as to cause a photon to scatter or reflect from the portion ofthe scene. The single-photon detector is in optical communication withthe phased array and detects the photon scattered or reflected by theportion of the scene. The processor is operably coupled to single-photondetector to estimate a distance between the at least one single photondetector and the portion of the scene based on a time of flight of thephoton.

In yet another example, an apparatus for imaging a scene includes atransmitter to illuminate at least one portion of the scene with aspatiotemporally varying interference pattern, an array of single-photondetectors to detect at least one photon reflected or scattered from theat least first one portion of the scene, and a processor, operablycoupled to the array of single-photon detector, to estimate a time offlight of the photon based on the spatiotemporally varying interferencepattern. The transmitter further includes a continuous wave (CW) lasersource to provide at least one continuous wave (CW) light beam. Thetransmitter also includes a phased array, in optical communication withthe at least one CW laser source, to generate the spatiotemporallyvarying interference pattern from the at least one continuous wave (CW)light beam. A first detector in the transmitter is in opticalcommunication with the phased array and measures an intensity at acenter of the spatiotemporally varying interference pattern. Acontroller, operably coupled to the first detector, applies a periodicphase setting, at a repetition rate substantially equal to or greaterthan 20 MHz, to the phased array and to change the periodic phasesetting of the phased array based at least in part on the intensitymeasured by the first detector.

BRIEF DESCRIPTION OF THE DRAWINGS

The skilled artisan will understand that the drawings primarily are forillustrative purposes and are not intended to limit the scope of theinventive subject matter described herein. The drawings are notnecessarily to scale; in some instances, various aspects of theinventive subject matter disclosed herein may be shown exaggerated orenlarged in the drawings to facilitate an understanding of differentfeatures. In the drawings, like reference characters generally refer tolike features (e.g., functionally similar and/or structurally similarelements).

FIG. 1 shows a schematic of an imaging system using a continuous wave(CW) light source and single-photon detectors.

FIG. 2 shows a schematic of an imaging system that can performmultiplexing of phased array maintenance and imaging.

FIG. 3 shows a timing diagram illustrating the multiplexing of phasedarray maintenance and imaging.

FIG. 4 shows a timing diagram illustrating methods of multiplexing ofphased array maintenance and imaging within one dither cycle.

FIG. 5A is a photograph of a target to be imaged in the far field.

FIG. 5B shows a 3D plot of range-resolved targets using systems andmethods illustrated in FIGS. 1-4.

FIG. 5C shows a contour plot of range-resolved targets using systems andmethods illustrated in FIGS. 1-4.

DETAILED DESCRIPTION

Overview

Pulsed LIDAR systems conventionally use laser pulses having a high peakpower to obtain range information. Recent progress in laser technologyhas yielded various semiconductor light sources. However, semiconductorlight sources normally have short upper-state lifetimes, which may limitenergy storage. In addition, high peak intensity may cause opticaldamage. Therefore, it can be challenging to use semiconductor lightsources for pulsed LIDAR applications.

Some LIDAR systems use continuous wave (CW) light sources to illuminatethe scene. These systems typically transmit a continuous signal, andranging can be carried out by modulating the intensity or frequency ofthe laser light. The travel time can then be derived from the phase orfrequency difference between the received and transmitted signals.Therefore, these LIDAR systems normally do not operate in single-photonmodes and the efficiency of photon usage is generally low.

To take advantage of the development of semiconductor lasers and thehigh efficiency of single-photon detection, methods and apparatusdescribed herein integrate an optical phased-array transmitter withsingle-photon detectors. An array of CW sources is used to effectivelypulse illuminate a target by electro-optically steering bright opticalintensity patterns in the far field. From the reference frame of a pointin the far field, a steered optical intensity pattern appears as atemporal pulse. Range information is thus obtained by measuring thepulse delay of the return to a pixel on a receiver.

For example, the optical phased array can include a one-dimensional (1D)transmitter array used for producing 1D fringes in the far field. In thearray direction, the periodicity of the array leads to bright and darkfringes. In the orthogonal cross-range direction, there is no variationin the fringes, resulting in a 1D periodic pattern of bright and darkstripes in the far field. A stripe is image relayed onto a column ofpixels in a 2D receiver array. As the fringes are steered, a column inthe far field is effectively pulse illuminated, and the time-of-flightto the image relayed column of pixels in the 2D array is measured. Asthe fringes continue to steer along the array dimension, the returns areimage relayed onto different columns in the 2D array. The photon arrivaltimes in the 2D receiver array allows us to generate 3D images using a1D array and a 2D receiver.

In this example, a CW semiconductor LIDAR system is built using anoptical phased array that can offer benefits such as broad spectralcoverage, high efficiency, small size, power scalability, andversatility. This system can enable the use of continuous-wave lasers toscale the power of the laser in a LIDAR application. In addition, thissystem can also entail the use of beam-steering to effectively pulseilluminate the scene by scanning an interference pattern (bright anddark bands) on and off a point in the scene. Advantages of this systeminclude: a) CW semiconductor lasers can be used which enables a broaderchoice of wavelengths (broader spectral coverage including eye-safewavelengths) with high-efficiency; b) the average power can be scaled byusing a phased array enabling use at long ranges; and c) thebeam-steering method enables arbitrary scanning in the far-field whilemaintaining high-brightness.

In addition, CW optical phased-array LIDAR systems also have goodversatility. The swept fringe illumination provides a direct detectmodality, enabling 3D imaging of the target. Alternatively, thisphased-array concept can be extended to fiber lasers to achieve highpower. Moreover, an optical phased array may be waveform modulated,enabling Doppler or laser communication in a coherent detection mode.For example, a frequency-chirped phase array may be used to determinethe velocity of the target using conventional Doppler detectiontechniques where the chirp in the return beam is provides velocityinformation. The multi-functionality of the phased array may thereforeeliminate the need for additional laser systems on a platform.Semiconductor sources also provide broad spectral coverage. Eye-safewavelengths may be achieved with increased efficiency over solid-statesources, which are typically pumped by semiconductor lasers and incur aquantum-defect penalty.

LIDAR Systems Including CW Light Sources and Single-Photon Detectors

FIG. 1 shows a schematic of an imaging system 100 that uses both CWlight sources and single-photon detection to image a scene. The system100 shown in FIG. 1 includes a phased array 110 to illuminate a scene 50with a time-varying optical intensity pattern 113 generated from acontinuous wave (CW) light beam 105. The illumination causes photons 115(also referred to as reflected photons, scattered photons, returnphotons, or return beam) to scatter or reflect from the illuminatedportion of the scene. The system 100 also includes at least onesingle-photon detector 120, in optical communication with the scene, todetect the photons 115 that are scattered or reflected by the portion ofthe scene. The system 100 also includes a processor 130 that is operablycoupled to the single-photon detector 120. In operation, the processor130 and estimates a distance between the single-photon detector 120 andthe portion of the scene based on a time of flight of the photons 115.

The time-varying optical intensity pattern 113 includes at least abright portion that can illuminate the scene and cause reflection orscattering of photons in the bright portion. From the point-of-view of apoint in the scene that is being illuminated, the time-varying opticalintensity pattern 113 appears as a temporal pulse, because the point isilluminated by the bright portion only for a finite period of timebefore the time-varying optical intensity pattern 113 changes itspattern. By image relaying the point in the scene onto the single-photondetector 120, the single-photon detector can also appear to be pulseilluminated, thereby allowing single-photon detection. The arrival timeof the pulse thus provides range information.

The Phased Array:

The phased array 110 in the system 100 receives the CW light beam 105and generates the time-varying optical intensity pattern 113 toilluminate the scene. In general, the phased 110 includes an array ofelements, each of which receives a respective portion of the CW lightbeam. Each element then applies phase modulation (e.g., a phase shift)to the received portion of the CW light beam 105. Sometimes the elementcan also apply intensity modulation to the received portion of the CWlight beam 105. At the output of the phased array 110, respectiveportions of the phased modulated CW light beam 105 can interact witheach other and form a desired intensity pattern (e.g., viainterference).

Applying different phase shifts to different portions of the CW lightbeam 105 received by the phase modulators can also modulate (alsoreferred to as “steer”) the resulting intensity pattern. The phasedarray 110 can steer the generated intensity pattern in at least twoways. In the first way, the phased array 110 can change the incidentdirection of the intensity pattern so as to sweep the intensity patternacross the scene to be imaged, i.e., the intensity pattern as a wholecan move around the scene. In the second way, the phased array 110 canchange the intensity distribution of the intensity pattern as if only aportion of the intensity pattern is scanning across the scene. Forexample, the intensity pattern can be a fringe pattern including aperiodical distribution of alternating bright and dark fringes within anarea. The phased array 110, by tuning the phase shift applied to eachportion of the CW light beam 105, can change the distribution of fringesas if they are rolling across the area (e.g., like a rolling escalator).Tuning the phase shift (and the speed of tuning) applied to each portionof the CW light beam 105 can also change the temporal period of thefringes (e.g., time interval for one fringe to illuminate a certainscene point twice). In this application, unless otherwise specified,beam steering by the phased array can be through either the first way,or the second way, or both.

In some examples, the phased array 110 includes a one-dimensional (1D)array of phase modulators (e.g., shown in FIG. 1). 1D phased arrays canproduce intensity patterns that vary in 1D, such as various types ofinterference fringes (including an interference pattern having a singlebright fringe). In some examples, the phased array 110 includes atwo-dimensional (2D) array of phase modulators. 2D phased arrays canproduce more complex and even arbitrary intensity patterns. Some 2Dpatterns may include a spot.

The phased array 110 can be constructed using various materials andphase modulation techniques. In some examples, the phased array 110includes an array of liquid crystal cells, which can be fabricated usingexisting techniques such as those used in liquid crystal displays.Applying a voltage over a liquid crystal can change the refractive indexof the liquid crystal, thereby changing the phase of light propagatingin the liquid crystal. Liquid crystals also have high birefringence, sothey can create a large optical path difference (OPD) between onepolarization and another polarization with modest voltages.

In some examples, the phased array 110 can be based on multiplexedvolume holography. In these examples, a limited number of gratings thatsteer to large angles can be written into a holographic recordingmedium. A particular grating can be addressed by small angle steering infront of the hologram. A second small angle steering device can be usedbehind the hologram to provide continuous angle steering between theangles produced by the holograms.

In some examples, the phased array 110 includes a birefringent prism. Inthese examples, a series of prisms can be used to steer to one of twostates depending on the polarization of the incident light.Electronically controlled waveplates can be used to alter thepolarization before each prism to choose the binary direction ofsteering. The phased array 110 can use fixed, modulo 2π,sawtooth-profile birefringent phase gratings. These gratings can providewide angle step-steering stages. The last wide-angle step-steeringapproach can be described as circularly polarized liquid crystalbirefringent polarization gratings (also referred to as liquid crystalpolarization gratings, or LCPGs). LCPGs can generate phase shift usingpolarization rotation.

In some examples, the phased array 110 can includemicroelectromechanical system (MEMS) devices. This MEMS approach canimplement a variable period approach to beam steering by fabricating aseries of mirrors that move perpendicular to the substrate, imparting apiston phase to light reflected off the surface.

In some examples, the phased array 110 can employ the electro-opticeffects in different materials, including lithium tantalite ((LiTaO₃),lithium niobate (LiNbO₃), magnesium-oxide-doped lithium niobate(MgO:LiNbO3), and Potassium titanyl phosphate (KTP) crystals. Therefractive index of these materials can be dependent on an appliedelectric field. Therefore, applying a voltage across the electrodes ofan electro-optic material can change the effective refractive index,thereby inducing the phase change as the light passes through thematerial. The applied voltage can be either direct current (DC) oralternating current (AC). AC voltage is normally used when periodicmodulation of the phase shift applied over the light is desired. Variousmodulation frequency (also referred to as drive frequency) can be used.In one example, the modulation frequency can be about 1 MHz to about 20GHz. In another example, the modulation frequency can be about 10 MHz toabout 5 GHz. In yet another example, the modulation frequency can be 100MHz to about 1 GHz. In yet another example, travelling wave electrodescan be used. In travelling electrodes, the electrical signal, whichapplies the voltage to change the refractive index of the electro-opticmaterials, propagates along the same direction as the optical wave does.Modulation frequency greater than 50 GHz can be achieved.

In some examples, arrays of waveguides can also be constructed using,for example, AlGaAs, for the beam steering of the phased array 110. Moredetails and examples can be found in P. F. McManamon, et al., A reviewof phased array steering for narrow-band electrooptical systems, Proc.IEEE 97, 1078-1096 (2009), which is incorporated herein by reference inits entirety.

In some examples, the system 100 uses a narrow-linewidth source, such asa semiconductor laser or a fiber laser, to seed an array ofslab-coupled-waveguide semiconductor amplifiers (SCOWAs). Each SCOWA iscapable of producing a diffraction-limited beam with power of up to 1 W.Dephasing occurring from non-common optical path drift between arrayelements can be mitigated by periodically toggling between a phase-lockcycle and a beam steering cycle. During the phase-lock cycle, the phasescan be synchronized via a stochastic-parallel-gradient-descent (SPGD)algorithm, which is a hill climbing-based algorithm requiring no phaseknowledge, no reference waveform, and only a single detector. During thebeam steering cycle, commercial LiNbO₃ phase modulators can be used tosteer the beam by applying a time-varying phase profile across theelements. Due to the power scalability of the SCOWAs and the fastresponse time of the phase modulators, this system can be readily scaledto multi-watt class output and GHz steering speed. More details can befound in W. R. Huang, et al., High speed, high power one-dimensionalbeam steering from a 6-element optical phased array, Optics Express, 20,17311 (2012), which is incorporated herein by reference in its entirety.

In some examples, the phased array 110 can include a two-dimensional(2D) array of phased modulators. For example, the phased array 110includes an optical phased array formed of a large number ofnano-photonic antenna elements which can project complex images into thefar field. The optical phased array, including the nano-photonic antennaelements and waveguides, can be formed on a single chip of silicon usingcomplementary metal-oxide-semiconductor (CMOS) processes. Directionalcouplers can evanescently couple light from the waveguides to thenano-photonic antenna elements, which emit the light as beams withphases and amplitudes selected so that the emitted beams interfere inthe far field to produce the desired pattern. In some cases, eachantenna in the phased array may be optically coupled to a correspondingvariable delay line, such as a thermo-optically tuned waveguide or aliquid-filled cell, which can be used to vary the phase of the antenna'soutput (and the resulting far-field interference pattern). More detailsof these example optical phased arrays can be found in U.S. Pat. No.8,988,754, which is incorporated herein by reference in its entirety.

The Single-Photon Detector:

The single-photon detector 120 in the system 100 shown in FIG. 1 candetect and/or record several types of information of the return photons115. In some examples, the single-photon detector 120 includes an arrayof detectors, each of which can be regarded as a pixel in the array. Inpractice, it can be helpful for the single-photon detector 120 to havethe following properties: 1) high detection efficiency, i.e., highprobability that a photon is successfully detected every time it hitsthe detector; 2) low dark current, i.e., low probability that thedetector registers a photon when none is there; 3) low reset or “deadtime”, i.e., a short interval after a detection during which the devicecannot detect a new photon; 4) low cross-talk, i.e., low probabilitythat neighboring pixels detect photons arising from the detectionprocess in a given pixel; and 5) low “timing jitter”, i.e., lowuncertainty in specifying when a photon arrives.

In one example, the single-photon detector 120 can include an array ofavalanche photodiodes (APDs), which are reverse-biased variants of p-njunction photodiodes. Typically, one pixel includes one APD, one biasingcircuit, one timing circuit, and an interface to the readout circuitry(e.g., shift registers) for the array. Without being bound anyparticular theory or mode of operation, reversely biasing a p-n junctionphotodiode can generate an electric field in the vicinity of thejunction. The electric field tends to keep electrons confined to the nside and holes confined to the p side of the junction. Absorption of aphoton having sufficient energy (e.g., >1.1 eV for silicon) can producean electron-hole pair. The electron in the electron-hole pair drifts tothe n side and the hole drifts to the p side, resulting in aphotocurrent flow in an external circuit.

The same principle also allows an APD to detect light. However, an APDis typically designed to support high electric fields so as tofacilitate impact ionization. More specifically, the electron and/or thehole in an electron-hole pair generated by photon absorption can beaccelerated by the high electric field, thereby acquiring sufficientenergy to generate a second electron-hole pair by colliding with thecrystal lattice of the detector material. This impact ionization canmultiply itself many times and create an “avalanche.” A competition candevelop between the rate at which electron-hole pairs are beinggenerated by impact ionization and the rate at which they exit thehigh-field region and are collected. The net result can be dependent onthe magnitude of the reverse-bias voltage: if the magnitude is below avalue (commonly known as the breakdown voltage), collection normallyoutruns the generation, causing the population of electrons and holes todecline. An APD operating in this condition is normally referred to as alinear mode APD. Each absorbed photon normally creates on average afinite number M (also referred to as the internal gain) of electron-holepairs. The internal gain M is typically tens or hundreds.

While M might be the average number of electron-hole pairs generated byone absorbed photon, the actual number may vary, inducing gainfluctuations. This gain fluctuation can produce excess noise, ormultiplication noise, which typically gets progressively worse with theincrease of M. Therefore, once the point is reached where themultiplication noise dominates over the noise introduced by downstreamcircuitry, further increases in gain may reduce the system'ssignal-to-noise ratio (SNR). The multiplication noise can also depend onmaterial properties because, in general, electrons and holes havedifferent likelihood to initiate impact ionizations. For example, in Si,electrons tend to be much more likely to impact ionize compared toholes. Therefore, it can be helpful for electrons to initiate impactionization in silicon-based APDs.

In another example, the single-photon detector 120 can include an APDoperating in Geiger mode (also referred to as a Geiger-mode APD orGmAPD). A GmAPD operates when the reversely biased voltage is above thebreakdown voltage. In this case, electron-hole pair generation normallyoutruns the collection, causing the population of electrons and holes inthe high-field region and the associated photocurrent to growexponentially in time. The growth of photocurrent can continue for aslong as the bias voltage is above the breakdown voltage.

A series resistance in the diode, however, can limit the current growthby increasing the voltage drop across the series resistance (therebyreducing the voltage across the high-field region) as the current grows.This effect can therefore slow down the rate of growth of the avalanche.Ultimately, a steady-state condition can be reached in which the voltageacross the high-field region is reduced to the breakdown voltage, wherethe generation and extraction rates balance. Stated differently, theseries resistance can provide negative feedback that tends to stabilizethe current level against fluctuations. A downward fluctuation incurrent, for example, can cause a decrease in the voltage drop acrossthe series resistance and an equal increase in the drop across the APDhigh-field region, which in turn increases the impact-ionization ratesand causes the current to go back up.

The quenching circuit of the APD employed for the system 100 can beeither passive or active. In a passive-quenching circuit, the APD ischarged up to some bias above breakdown and then left open circuited.The APD then discharges its own capacitance until it is no longer abovethe breakdown voltage, at which point the avalanche diminishes. Anactive-quenching circuit actively detects when the APD starts toself-discharge, and then quickly discharges it to below breakdown with ashunting switch. After sufficient time to quench the avalanche, theactive-quenching circuit then recharges the APD quickly by using aswitch. In LIDAR systems, where the APD typically detects only once percycle, the recharge time can be slow. There is also interest, however,in using the Geiger-mode APDs to count photons to measure optical fluxat low light levels. With a fast active-quenching circuit, the APD canbe reset shortly after each detection (e.g., on a time scale as short asnanoseconds), thereby allowing the APD to function as a photon-countingdevice at much higher optical intensities.

In yet another example, the single-photon detector 120 can include anarray of superconducting nanowire single-photon detectors (SNSPDs), eachof which typically includes a superconducting nanowire with arectangular cross section (e.g., about 5 nm by about 100 nm). The lengthis typically hundreds of micrometers, and the nanowire can be patternedin compact meander geometry so as to create a square or circular pixelwith high detection efficiency. The nanowire can be made of, forexample, niobium nitride (NbN), tungsten silicide (WSi), YBa₂Cu₃O_(7-δ),or any other material known in the art.

In operation, the nanowire can be maintained below its superconductingcritical temperature Tc and direct current biased just below itscritical current. Without being bound by any particular theory of modeof operation, incident photons having sufficient energy to disrupthundreds of Cooper pairs in a superconductor can therefore form ahotspot in the nanowire. The hotspot itself typically is not largeenough to span the entire width of the nanowire. Therefore, the hotspotregion can force the supercurrent to flow around the resistive region.The local current density in the sidewalks can increase beyond thecritical current density and form a resistive barrier across the widthof the nanowire. The sudden increase in resistance from zero to a finitevalue generates a measurable output voltage pulse across the nanowire.

Various schemes can be employed in SNSPD to improve the detectionperformance. In one example, the SNSPD can employ a large area meanderstrategy, in which a nanowire meander is written typically across a 10μm×10 μm or 20 μm×20 μm area so as to increase the active area andthereby improve the coupling efficiency between the incident photons andthe SNSPD. In another example, the SNSPD can include a cavity andwaveguide integrated design, in which a nanowire meander can be embeddedin an optical cavity so as to further increase the absorptionefficiency. Similarly, a nanowire can be embedded in a waveguide so asto provide a long interaction length for incident photons and increaseabsorption efficiency. In yet another example, ultra-narrow nanowires(e.g., 20 nm or 30 nm) can be employed to construct the nanowire meanderso as to increase the sensitivity to low-energy photons.

In yet another example, the single-photon detector 120 can include atransition edge sensor (TES), which is a type of cryogenic particledetector that exploits the strongly temperature-dependent resistance ofthe superconducting phase transition. In yet another example, the focalplane array 130 can include a scintillator counter which can detect andmeasure ionizing radiation by using the excitation effect of incidentradiation on a scintillator material, and detect the resultant lightpulses.

In some examples, the single-photon detector can include a 32×32 GM-APDarray, which uses a continuous read-out integrated circuit (ROIC) torecord the arrival time of incident photons relative to a 500 MHz clock.Once powered, the APD array can be in an armed state and can be ready tofire on an incident photon. Once an avalanche event has been triggeredby an arriving photon, a quench circuit can be activated to quench theAPD. A reset time to rearm the APD pixels is about 2 μs.

Typically, the GM-APD array operates in a regime with less than onephotoelectron on a pixel per reset time. Since the arriving photons canbe described by Poisson statistics, the arrival of two or more photonson a pixel during a reset time can be unlikely. Arrival of two or morephotons on a pixel may blind a pixel to the arrival of subsequentphotons due to the finite reset time associated with registering thearrival of the first photon. In this example, the input to the receivercan be significantly attenuated.

High temporal resolution may be obtained by matching the receiver arrayto the transmitter. For illustrating purposes only, a filled aperturearray including N top-hat array sub-aperture elements with width w andperiod P=w can be employed such that the far field includes a singlefringe. For a filled-aperture array, there can be N spatially resolvablefringe positions within the far field. Similarly, there can be Ntemporally resolvable pulse intervals, time and space being related bythe fringe velocity (t=x/v). It therefore can be desirable to have atleast as many pixels in one dimension of the receiver as there arenumber of elements in the transmitter to temporally or range resolve afar-field object. The total number of elements in the receiver array cantherefore scale as ˜N², with a fringe width matched to a pixel width onthe receiver. The receiver scaling can be generalized to a non-filledaperture array with multiple fringes. For M fringes, each with N pixelsalong a dimension of the array, the total number of pixels in the arraycan scale as (MN)² in order to match the receiver field of view (FOV) tothe transmitter.

There can be tradeoffs between a filled and non-filled aperturetransmitter with the same sub-aperture w and far-field envelope (λR/w).The non-filled aperture can generally have a larger period and arrayaperture (NP). The cross-range resolution ΔR_(c)=λR/NP can improve withthe increased array aperture, but may at the expense of more receiverpixels. A filled aperture can allow for efficient matching of a fractionof the transmit envelope to a receiver FOV by restricting the scan rangeof a single fringe. For a non-filled aperture, matching the receiver FOVto a fraction of the transmit envelope may result in some efficiencyloss, since not all of the transmitted fringes are utilized. In thisexample, about 3 of the 42 fringes are utilized in generating a 3Dimage. A better matched receiver to the transmit envelope can improvethe system efficiency.

The Processor:

The processor 130 in the system 100 is operably coupled to thesingle-photon detector 120 and processes data generated by thesingle-photon detector 120. In particular, the processor 130 canestimate a distance between the single-photon detector 120 and theportion of the scene based on a time of flight of the return photons115. The processor 130 can be coupled to the single-photon detector 120via a Readout Integrated Circuit (ROIC) (not shown in FIG. 1). Inoperation, the ROIC can read out the photocurrent generated by thesingle-photon detector 120, time stamp the photon arrivals, read out thepixel locations of the received photons, and convey the information offthe ROIC and into the processor 130.

In some examples, the processor 130 can include either a FieldProgrammable Gate Array (FPGA) or an Application Specific IntegrationCircuit (ASIC). In one example, the FPGA approach can be employed forits relatively simple design. An FPGA is generally a semiconductordevice containing programmable logic components conventionally referredto as “logic blocks” and programmable interconnects. Logic blocks can beprogrammed to perform the function of basic logic gates such as AND, andXOR, or more complex combinational functions such as decoders ormathematical functions. For example, a VIRTEX-7 FPGA, manufactured byXILINX, can deliver 2 million logic cells, 85 Mb block RAM, and 3,600DSP48E1 slices for new possibilities of integration and performance.

In another example, the ASIC approach can be employed for its often morepowerful computation capability. An ASIC is generally an integratedcircuit designed for a particular use, rather than intended forgeneral-purpose use. Example ASICs include processors, RAM, and ROM.Therefore, ASIC can be more complicated, and thus more expensive, thanFPGA. In practice, the decision of using a FPGA or an ASIC may depend onseveral factors, such as the budget, time constraint, and fabricationcapabilities.

In yet another example, the processor 130 in the system 100 can be amicroprocessor, microcontroller, or CPU. In yet another example, theprocessor 130 may be any electronic device that can analyze and/orprocess data and generate one or more 3D images. To this end, theprocessor 120 may include or be associated with a computing device, suchas a portable computer, personal computer, general purpose computer,server, tablet device, a personal digital assistant (PDA), smart phone,cellular radio telephone, mobile computing device, touch-screen device,touchpad device, and the like.

Sources of CW Light Beams:

The system 100 shown in FIG. 1 further includes a CW light source 140that provides the CW light beam 105 for the phased array 110. A seeddistributor 150 (e.g., beam splitter, directional couplers, etc.) can bedisposed between the light source 140 and the phased array 110 so as tosplit the CW light beam 105 into multiple portions, each of which can bereceived by one element in the phased array 110. In addition, eachelement in the phased array 110 can be further coupled to a respectiveamplifier in an amplifier array 160. The amplifier array 160 can amplifythe modulated light beam from the phased array 110 so as to increase theoverall brightness of the beam. The amplifier array 160 can also applydifferent amplification ratios to different portions of the CW lightbeam 105 so as to, for example, improve the quality of the resultingintensity pattern (e.g., in contrast and/or uniformity).

In some examples, the light source 140, the seed distributor 150, thephase array 110, the amplifier array 160 are optically coupled to eachother via free space. In some examples, the optical communication amongthese components can be implemented using waveguides, including but arenot limited to, fibers or semiconductor waveguides. In some examples,the optical communication can use a combination of free space andwaveguides.

The light source 140 can use various types of light sources. In oneexample, the light source 140 can include a fiber laser, which typicallyhas good spatial and spectral qualities and can be configured to operatein continuous, modulated, or pulsed mode. The output wavelength of afiber laser may be tunable and can be eye-safe. The core of the fibercan be doped with one or more rare-earth elements, such as erbium (Er),ytterbium (Yb), neodymium (Nd), dysprosium (Dy), praseodymium (Pr),thulium (Tm), holmium (Ho). Nd- or Yb-doped silica fiber provideemission around 1 μm. Yb-doped silica fiber can be a promising platformfor high power applications due to the high optical to opticalconversion efficiency. Er-doped silica fiber lasers and amplifiers canoperate at around 1.55 μm. Emission at 2 μm can be achieved by thuliumor holmium-doped silica or germanate fibers.

In another example, the light source 140 can include a semiconductorlaser. The semiconductor laser can produce diffraction-limited emissionby, for example, a ridge waveguide having a width of about severalmicrons so as to preferably lase (amplify via stimulated emission) thefundamental mode. The semiconductor laser can also produce spectrallystabilized emission using a Bragg grating integrated into thesemiconductor chip so as to construct a distributed Bragg reflector(DBR) laser or a distributed feedback (DFB) laser. Semiconductor opticalamplifiers, either monolithically or hybrid integrated with the masteroscillator, can be used to increase the output energy of the laser. Theamplifiers can be constructed in a multi-stage configuration, in whichcase the amplifiers can also be employed to control the repetition rate,either as pulse picker to select individual pulses or as optical gate togenerate an optical pulse out of a continuous wave (CW) masteroscillator with desired spectral properties.

In yet another example, the light source 140 can include a semiconductorlaser based on based on two InGaAsP/InP monolithic Master OscillatorPower Amplifiers (MOPAs) operating at, for example, about 1.57 μm. EachMOPA can include a frequency stabilized Distributed Feedback (DFB)master oscillator, a modulator section, and a tapered amplifier. The useof a bended structure (the oscillator, the modulator, and the amplifierare not arranged along a straight line) may avoid undesired feedback andthus provide good spectral properties together with high output powerand good beam quality.

In yet another example, the light source 140 can include a parametriclight source such as an optical parametric oscillators (OPOs) and/or anoptical parametric amplifier (OPAs), which are typically tunable and cangenerate emission at wavelengths from the ultraviolet to themid-infrared range. OPOs and OPAs can have the benefit of good spectralproperty, high electrical-to-optical efficiency, ruggedness, and smallvolume.

The Feedback System:

The system 100 further includes a beam splitter 165 disposed at theoutput of the phased array 110 so as to direct part of the near fieldtime-varying optical intensity pattern 111 toward a photodetector 180. Acollecting lens 182 can be used to collect light of the near fieldtime-varying optical intensity pattern 111 and a pinhole or otherspatial filter 184 can be used to control which portion of the nearfield time-varying optical intensity pattern 111 is to be monitored bythe photodetector 180. In one example, the photodetector 180 can monitorthe intensity of the center portion of the near field time-varyingoptical intensity pattern 111. In another example, the photodetector 180can monitor any other portions of the near field time-varying opticalintensity pattern 111. The detected intensity can then be transmitted toa phase controller 190, which, based on the detected intensity, canadjust the phase shift applied to each portion of the CW light beam 105in the phase modulator 110. Through this feedback system including thedetector 180 and the phase controller 190, the phase array can generatethe time-varying optical intensity pattern 113 with desired properties,including brightness, spacing, contrast, or any other propertiesapplicable here.

Methods of Imaging Using CW Light Beams and Single-Photon Detection

FIG. 2 shows a transmitter 200 that can be used with a receiver to imagea scene using CW light beams and single-photon detection. Thetransmitter 200 includes a light source 240 that delivers a CW lightbeam toward an isolator 245, which prevents light (e.g., reflections)from feeding back into the light source 240. After the isolator 245, theCW light beam is split into multiple portions by a fiber splitter 250 soas to feed a phased array 210 that includes multiple phase modulators.Modulators in the phased array 210 modulate the different portions ofthe CW light beam to create a temporally varying optical intensitypattern. An array of amplifiers (for example fiber, solid state, orsemiconductor amplifiers) 260 then amplifies the output of the phasedarray 210. A micro-lens array 262 disposed at the output of the fiberamplifiers 260 increases a fill factor of the fiber amplifiers 260.

A beam splitter 265 directs part of the temporally varying opticalintensity pattern toward a detector 280, which is disposed behind acollecting lens 282 and a pinhole 284. The detector signal from thedetector 280 is transmitted to a phase controller 290 via a first switch292. The phase controller 280, based on the detected signal, controlsthe phases applied to the phased array 210 so as to improve the qualityof the resulting temporally varying optical intensity pattern. Thetransmitter 200 also includes a wavefront profile controller 270 that isoperably coupled to the phased array 210 via a plurality of secondswitches 272 (collectively referred to as second switch 272). Each phasemodulator in the phased array 210 can be connected to a respectivechannel, via a respective switch in the second switch 272, in thewavefront profile controller 270 so as to allow independent control overeach phase modulator.

With reference to, but not limited to, the systems 100 shown in FIG. 1and the transmitter 200 shown in FIG. 2, a method of imaging a sceneincludes generating a temporally varying optical intensity pattern fromat least one continuous wave (CW) light beam (e.g. using the phasedarray). The generated temporally varying optical intensity pattern isthen used to illuminate at least one portion of the scene so as to causea photon to scatter or reflect off the at least one portion of thescene. A single-photon detector is then used to detect the photonreflected or scattered from the at least one portion of the scene. Themethod then estimates a distance between the single-photon detector andthe at least one portion of the scene based on the temporally varyingoptical intensity pattern and a time of flight of the detected photon.

Generation of Temporally Varying Optical Intensity Patterns:

The temporally varying optical intensity pattern can convert a CW lightbeam into a virtual “pulse” so as to illuminate the scene and allowranging based on single-photon detection. The temporally varying opticalintensity pattern can be generated using a phased array according tovarious methods. One of the methods can be a stochastic parallelgradient descent (SPGD) technique, which can be implemented using thesystem shown in FIG. 2, when the first switch 292 is closed, i.e., whenthe detector 280 is connected to the phase controller 290.

The initial alignment of the phased array can start with applying afirst phase dither δD (e.g., with a magnitude of about 2π/40) having afirst sign on the phased array and measuring a first intensity of acenter portion of the temporally varying optical intensity pattern. Themethod then applies a second phase dither −δD having a second signopposite to the first sign and measuring a second intensity of thecenter portion of the temporally varying optical intensity pattern. Aslope S of an intensity change can be calculated using the firstintensity and the second intensity. Using the slope S, an updated phasecorrection Δφ₂ can be calculated as Δφ₂=Δφ₁+S*δD, where Δφ₁ is the phaseshift applied by the phased array before applying the first phase ditherδD. At the very beginning, the phase shift can be a random phase shift.This dither-measurement-update cycle can be repeated until the slope Sis substantially close to zero. In general, the first phase dither δDand the second phase dither −δD have the same magnitude but oppositesigns.

The first phase dither δD, the second phase dither −δD, the slope S, thefirst phase shift Δφ₁, and the second phase shift Δφ₂ are all vectorquantities including an array of elements, each of which is a respectivephase dither or phase shift applied to a corresponding phase modulatorin the phased array. In one example, the phased array includes N phasemodulators distributed in a one-dimensional array, and the first phaseddither δD is also a one-dimensional array δD=[δD₁, δD₂, δD₃, . . . ,δD_(N)]. In another example, the phased array includes M×N phasemodulators distributed in a two-dimensional array (i.e., a matrix), thenthe first phase dither δD can include an array of elements δD=δD(i, j),wherein i is from 1 to M and j is from 1 to N.

Steering of Temporally Varying Optical Intensity Patterns:

When imaging the scene, the phased array can steer the temporallyvarying optical intensity pattern. As introduced before, the steeringcan include sweeping the optical intensity pattern across the scene orvarying the optical intensity pattern (e.g., rolling the fringes of aninterference pattern). In one example, a method of steering thetemporally varying optical intensity pattern includes applying asteering phase shift “(N-1)Δϕ” on the Nth phase modulator in a phasedarray that includes an array of phase modulators. In other words, thesteering phase shifts applied to the modulators in the phased arraysteps in increments of Δϕ, e.g., [0; Δϕ; 2Δϕ; 3Δϕ; . . . ; (N-1)Δϕ].Then the basic phase shift Δϕ can be varied from 0 to 2π so as to sweepat least a portion of the temporally varying optical intensity patternacross the at least one portion of the scene. For example, varying Δϕfrom 0 to 2π for a fringe pattern can shift the fringe pattern in onedimension by one fringe. The pattern after steering and before thesteering can have the same intensity distribution, but a point in thescene is illuminated by one fringe for a finite time (i.e., the time itmove the fringe by one fringe width). Then the process can repeat formultiple times to as to collect more photons for imaging.

The time it takes to complete a cycle of varying Δϕ can be dependent onthe material of the phase modulator, the electronics in the phasecontroller, and/or the data collection capacity of the detector. Thistime is also referred to as modulation time, and the inverse ofmodulation time is conventionally referred to as modulation frequency.In one example, the phase modulator can include liquid crystals and themodulation frequency can be greater than 1 KHz (e.g., about 1 KHz toabout 10 KHz). When using other phase modulators, such as electro-opticmodulators, the modulation frequency can be greater than 1 MHz (e.g.,about 1 MHz to about 20 GHz, about 10 MHz to about 1 GHz, or about 100MHz to about 500 MHz). The modulation time can affect the temporalresolution of the resulting LIDAR system. In general, a shortermodulation time can lead to a higher temporal resolution, since thevirtual pulse simulated by the CW light beam has shorter effective pulseduration.

In some examples, the temporally varying optical intensity pattern caninclude a fringe pattern generated by a system including a masteroscillator split into N beams to feed an optical phased array (e.g.,shown in FIG. 2). The fringes in the far field are consistent with theFourier transform of the near field, and the number of fringes isinversely proportional to the fill factor, which is a ratio of theemitter aperture (w) to the array period (P). Typically, a microlensarray is used to increase the fill factor and reduce the number offringes.

The fringe(s) in the far field can be scanned by applying an incrementalphase shift [0; Δϕ; . . . (N-1)Δϕ] to each array element where N is thenumber of elements within the array. To scan a full inter-fringe spacing(i.e., to steer a fringe to its neighboring fringe position), the phaseshift Δϕ can be varied by 2π. The dwell time of a fringe on a point inthe far field can be given by:τ=W/v  (1)where W is the width of the fringe at range R and v is the scanvelocity. The width of a far-field fringe is approximately W≈λR/(NP),and Λ≈λR/P is the separation between fringes. The phased-array aperturecan scale linearly with the number of elements N, and the far-fieldfringe width therefore can scale inversely W ∝ (1/N) with the arraycount. If the array elements are phase modulated to scan the far-fieldinter-fringe spacing Λ in a time T, the dwell time becomesτ=T/N=1/(fN)  (2)

The dwell time (pulse duration) generally scales inversely with thenumber of elements and can become shorter with faster drive frequencies(f=1/T). The range resolution can be given by ΔR=cτ/2, where c is thespeed of light, and the factor of two accounts for the round trip. UsingEquation (2), the range resolution may be expressed as:ΔR=c/(2Nf)  (3)

The unambiguous range can be determined by the repetition rate of thefringes. Since the fringes are swept past the target in a periodicfashion, it can be challenging to unambiguously determine that a returnis associated with a particular period of the repetitive scan. Similarto conventional LIDAR systems, the unambiguous range may be expressedas:R_(u) =c/(2f)  (4)where the factor of 2 accounts for a round trip. The number ofresolution bins ΔR within an unambiguous range R_(u) can be proportionalto the number of elements in the array. Applying conventional radartechniques can increase the unambiguous range, e.g., the repetition ratemay be varied among subsequent scans.

In some examples, a near-unity fill factor can be used to result inapproximately one fringe. A typical system may have more than 100elements operating at sweep rates of f˜20 MHz resulting in a ΔR˜7.5 cmresolution. In the example illustrated here, a six-element opticalphased array with a range resolution ΔR˜0.5 m is used and thecorresponding unambiguous range is R_(u)=7.5 m.

The emitter aperture in this example includes a w=6 μm modal fielddiameter with a period of P=250 μm, generating approximately 42 fringesin the far field. The relative phase shift between elements can besinusoidally modulated at f=20 MHz with a relative phase function Δϕ=πsin(2πft). For the sinusoidal scan, the fringe position varies accordingto y(t)=(λR/2P) sin (2λft). The average magnitude of the velocity istherefore v=2λR/(PT). This average velocity in Eq. (1) can be used toyield an average pulse width of τ=(1/2fN)=3.6 ns in this example. Thispulse duration can result in an average range resolution of ΔR≈0.5 m. Insome examples, linear-phase functions may be used to achieve constantvelocity fringes and therefore constant pulse widths.

Multiplexing of Phased Array Alignment and Steering:

The initial alignment of the phased array can generate the temporallyvarying optical intensity patterns with desired properties (e.g., a highbrightness). During imaging, this initial alignment may gradually fade.Therefore, it can be beneficial to check the alignment of the phasedarray and make corrections to the alignment (also referred to asmaintenance) during imaging. Maintenance can be carried out using SPGDmethods similar to the methods for initial alignment. Maintenance can bemultiplexed with steering of the temporally varying optical intensitypattern without disrupting the streamlined imaging process.

Example methods of multiplexing maintenance and steering can beillustrated with respect to FIGS. 2-4. In FIG. 2, if the first switch292 (switch A) is closed and the second switch 272 (switch B) is open,the system 200 can perform initial alignment of the phased array 210. Asdescribed above, in initial alignment, a positive dither can be appliedto a correction based on a previous dither cycle followed by ameasurement of the intensity of the temporally varying optical intensitypattern in response to the dither. Then a negative dither can be appliedand a second measurement is performed. A correction is then calculatedbased on the first measurement and the second measurement. Thecalculated correction is then applied during the start of the nextcycle. This process can repeat during the entire imaging process so asto, for example, maintain a high brightness of the temporally varyingoptical intensity pattern. If the phase dither and the subsequentmeasurements can be fast enough, the time between dithers can be used tosteer the temporally varying optical intensity pattern for imaging.

FIG. 3 shows a timing diagram 300 of the first switch 292 and the secondswitch 272 so as to illustrate multiplexing of phased array maintenanceand beam steering. The timing diagram 300 shows three dither cycles,each of which can be further divided into a first section 310 and asecond section 320. In the first section 310 (also referred to asbore-sighting section), the first switch 292 is closed and the secondswitch 272 is open. During bore-sighting section 310, phase dithers canbe quickly applied to the phased array 210 and measurements of thetemporally varying optical intensity pattern by the detector 280 inresponse to the phase dither are also carried out. This dither andmeasurement can correct misalignment of the phased array and maintain adesirable brightness of the resulting temporally varying opticalintensity pattern. In the second section 320 (also referred toillumination window), the first switch 292 is open and the second switch272 is closed. During this illumination window 320, the phased array ismodulated so as to steer the temporally varying optical intensitypattern so as to carry out the imaging operation.

The frequency of the dither cycles can depend on the time it takes forthe phased array to be significantly away from its initial alignment(e.g., when the brightness of the resulting temporally varying opticalintensity pattern is only half of the peak value). For example, withoutany maintenance, it may take the phased array 100 ms to misalignsignificantly. Then the dither frequency can be set at around 1 KHz (onedither every 1 ms) to allow multiple dither cycles to correct thepotential alignment within the 100 ms time period. In some examples, thedither frequency can be about 100 Hz to about 100 KHz. In some examples,the dither frequency can be about 1 KHz to about 10 KHz.

The percentage of the first section 310 in one dither cycle can bedependent on the speed of the electronics for phase modulation and datacollection. The electronics is can be the circuitry to apply themodulation signal for the phased array 210 and to collect the intensitydata of the resulting time-varying optical intensity pattern in responseto the modulation signal. In general, faster electronics can allow asmaller percentage of the first section in each cycle and thereforereduce the interference with the beam steering. In some examples, thepercentage can be about 0.1% to about 10% of the dither cycle, in whichcase about 90% to about 99.9% of the cycle time can be used for beamsteering and imaging. In some examples, the percentage can be about 1%to about 2% of the dither cycle. In some examples, the first section canbe completed within 50 ns. In some examples, the first section can becompleted within 10 ns. In some examples, the first section can becompleted within 5 ns. With the development of fast electronics, thefirst section can be arbitrarily short so as to further reduce theinfluence on the imaging operation.

FIG. 4 shows a timing diagram 400 of two dither cycles to illustratemore specific steps that can be performed in the multiplexing of phasedarray maintenance and beam steering. The two dither cycles include sixsections (section 410 to section 460). In section 410, an up-dither(“+δD”) is applied to the phased array 210, and a subsequent measurementis performed in section 420. In the remainder of the time in the firstdither cycle (section 430), arbitrary steering can be performed byturning on the waveform generation while turning off the phasemaintenance (in FIG. 2, the second switch 272 is closed while the firstswitch 292 is open). At the end of the first dither cycle, the secondswitch 272 is opened and the first switch 292 is closed to perform adown dither (“−δD”) in section 440 and the corresponding measurement insection 450. Then the phased array can again be used for beam steeringin section 460.

In some examples, the dither and measurement (first section) in a dithercycle can be performed without adjusting the first switch 292 and thesecond switch 272. More specifically, during steering, the variablephase shift Δϕ in the phase shift [0; Δϕ; 2Δϕ; 3Δϕ; . . . ; (N-1)Δϕ]applied over the phased array varies between 0 and 2π. The dither andmeasurement can then be performed when the variable phase shift Δϕ issubstantially close to zero in order to reduce or eliminate the effectof the phase dither on the steering.

In some examples, the temporally varying optical intensity patterninclude a fringe pattern created by a 1D array of phase modulators(e.g., shown in FIG. 2). The method of beam steering can include an openloop arbitrary steering function onto the array which allows the arrayto generate arbitrary patterns. Periodically, and on times of smallduration relative to the period, the arbitrary steering function can beforced to return to zero (a bore sight location) in order to allow SPGDto dither the phases or perform measurements. For the remainder of thetime interval, the array can be free to steer in an arbitrary fashionwithout impacting the performance of the phase-control algorithm. Sincethe return to zero time can be made small relative to the duration ofthe scan (for example <1% of the scanning time), the scanning appears tobe continuous with minimal disruption as a result of the <1% phasemaintenance time.

The SPGD method, also referred to as a hill climbing method, is used tophase the array such that it generates high-contrast fringes in the farfield. SPGD can be an adaptive control method used to optimize theon-axis intensity in the far field. The advantage of SPGD can be that itdoes not require direct knowledge of the individual phases of the arrayelements. The array output can be sampled by placing a pinhole in frontof a detector. The pinhole can be much smaller than the central lobewidth (ΔW≈(λF/NP)) of the fringe pattern, where F is the transform lensfocal length used to generate the far field.

A random phase dither (δD) can be applied to the array elements followedby measurements of the response. Subsequently, the same dither can beapplied with an opposite sign (−δD). Based on the response to the twodither measurements, a slope (S) of the measured change in intensity tothe applied phase dither can be determined. A correction (Δϕ_(new)) tothe initial phase distribution across the array (Δϕ_(old)) can besubsequently applied, which includes a summation of the initial phasedistribution and the phase dither multiplied by the measured slope(Δϕ_(new)=Δϕ_(old)+SδD). This process can continue iteratively until theslope becomes zero, in which case a maximum is obtained, and no furthercorrections are applied (Δϕ_(new)=Δϕ_(old)). At the end of this cycle,the optimal phase offsets can be held fixed followed by an open-loopsteering cycle in which a relative phase difference [0; Δϕ; . . . ;(N-1)Δϕ] is applied to the array elements.

The open-loop steering can be performed during a time interval shorterthan the dephasing time of the array, which can be dependent on theintrinsic phase noise and environmental disturbances (typically ˜300 msin this example). After the beam-steering cycle, the steering phaseoffsets are nulled, and the SPGD method can be resumed. The convergencetime of the SPGD method, on average, can be given byτ_(c)=(k2(N-1)/f_(d)), where k is a constant between 2 and 4, and f_(d)is the dither frequency. In this example, the dither frequency isf_(d)=2 kHz. Conservatively, a 100 ms time interval can be sufficient toachieve SPGD convergence for phase maintenance followed by a 100 msopen-loop steering cycle. Alternative SPGD implementations are alsopossible, which may allow for continuous steering.

The SPGD method includes dithering the phases of the array elements,measuring the response to the dither, and implementing a correction tooptimize the measured fringe signal on a detector. Steering the phasedarray includes adding a steering phase function to the array elements.If the steering phase function occurs simultaneously with the ditherduring a measurement, the measured response to the dither may beinterfered by the presence of the steering phase offset. However, thesteering phase function has periodic zero crossings. The SPGDmeasurements can be synchronized to sample the SPGD detector duringnarrow windows centered on the zero crossings of the steering phasefunction. In this case, interference between the beam steering and theSPGD measurements can be reduced or eliminated. Equivalently, one mayvisualize a steered fringe periodically sweeping past the detector.Sampling the fringe during the time intervals corresponding to thefringe overlap with the detector results in strobing the fringe. Fromthe sampled SPGD detector point of view, the fringe appears stationary.

Estimation of Three-Dimensional (3D) Images of the Scene:

Data collected by the single-photon detectors can be used to construct3D images of the scene. The raw data collected by single-photondetectors is normally referred to as point clouds. A point cloud caninclude a plurality of data points, each of which has a two dimensionalcoordinate (x, y) and a corresponding range information z derived fromthe time-of-flight of the photon that creates this data point. Thearrival times of the pulses on the receiver can be compared to the timesat which the transmitter was pointing a bright portion of thetime-varying optical intensity pattern at the corresponding imagerelayed location in the far-field so as to derive the time of flight ofthe photons. Furthermore, in the case of Geiger-mode APD receivers, aparticular point in the scene can be illuminated for multiple times.Correspondingly, a particular return (beam path) from the scene point inthe far field to a receiver pixel from a point in the far-field can alsooccur for multiple times in order to obtain sufficient signal relativeto the noise on the receiver. Histograms may be generated to count allthe returns in a particular time bin relative to the repetition rate ofthe returns.

In some examples, fringe patterns are used to emulate illuminating thescene with pulses of light. In these examples, each pixel on thereceiver sees one fringe, which appears to sweep across the scene. Eachpixel coordinate (x₀, y₀) can be mapped to an angle (θ_(x), θ_(y)). Thesweeping fringe appears to pulse illuminate an angle (θ_(x), θ_(y),),i.e., one angle in the fringe sweep is mapped onto a pixel. The time thefringe was pointed at angle (θ_(x), θ_(y)) can be recorded as time to.The detector can also record the time at which it receives a return(referred to as time t). The time of flight is the t-t₀. The range canbe determined from r=c(t-t₀)/2.

Images Obtained with CW Light Beams and Single-Photon Detection

FIGS. 5A-5C show experimental results of an example implementation ofthe LIDAR system shown in FIG. 1. FIG. 5A shows a photograph of thetargets. FIG. 5B shows a 3D image obtained with the LIDAR system, andFIG. 5C shows a 2D range-resolved contour plot derived from the LIDARdata. Four objects are placed at three different image planes. Thenominal range to the target complex is approximately 16 m. The resolvedobject range is accurate to within the unambiguous range interval of 7.5m. The distance between the first target plane (circular object labeledA) and the second plane (rectangle labeled B) is inferred to be 1.0 m,while the distance between the second and third target plane (objectpair labeled C: rectangle and MIT Lincoln Laboratory logo) is estimatedto be 1.6 m. These distances are consistent with the spacing betweenobjects in the laboratory (1.0 and 1.5 m, respectively).

Comparing the time of arrival of the photon at a given pixel relative tothe time when the phased array is pointing at a given pixel provides therange information, resulting in the range-resolved images shown.Specifically, histograms of the arrival times of the photons can begenerated. A mean time of arrival is assigned to the peaks in thehistogram. Several pulses can be used to integrate the returns whilemaintaining a good signal-to-dark-count ratio. The cross-rangeresolution ΔR_(c)=λR/NP can be limited by the transmitter array apertureNP in order to match a pixel on the receiver to a fringe width. In thisexample, the cross-range resolution is ΔR_(c)≈9 mm at the 16 m distanceto the target complex.

Conclusion

While various inventive embodiments have been described and illustratedherein, those of ordinary skill in the art will readily envision avariety of other means and/or structures for performing the functionand/or obtaining the results and/or one or more of the advantagesdescribed herein, and each of such variations and/or modifications isdeemed to be within the scope of the inventive embodiments describedherein. More generally, those skilled in the art will readily appreciatethat all parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the inventive teachingsis/are used. Those skilled in the art will recognize, or be able toascertain using no more than routine experimentation, many equivalentsto the specific inventive embodiments described herein. It is,therefore, to be understood that the foregoing embodiments are presentedby way of example only and that, within the scope of the appended claimsand equivalents thereto, inventive embodiments may be practicedotherwise than as specifically described and claimed. Inventiveembodiments of the present disclosure are directed to each individualfeature, system, article, material, kit, and/or method described herein.In addition, any combination of two or more such features, systems,articles, materials, kits, and/or methods, if such features, systems,articles, materials, kits, and/or methods are not mutually inconsistent,is included within the inventive scope of the present disclosure.

The above-described embodiments can be implemented in any of numerousways. For example, embodiments of designing and making the technologydisclosed herein may be implemented using hardware, software or acombination thereof. When implemented in software, the software code canbe executed on any suitable processor or collection of processors,whether provided in a single computer or distributed among multiplecomputers.

Further, it should be appreciated that a computer may be embodied in anyof a number of forms, such as a rack-mounted computer, a desktopcomputer, a laptop computer, or a tablet computer. Additionally, acomputer may be embedded in a device not generally regarded as acomputer but with suitable processing capabilities, including a PersonalDigital Assistant (PDA), a smart phone or any other suitable portable orfixed electronic device.

Also, a computer may have one or more input and output devices. Thesedevices can be used, among other things, to present a user interface.Examples of output devices that can be used to provide a user interfaceinclude printers or display screens for visual presentation of outputand speakers or other sound generating devices for audible presentationof output. Examples of input devices that can be used for a userinterface include keyboards, and pointing devices, such as mice, touchpads, and digitizing tablets. As another example, a computer may receiveinput information through speech recognition or in other audible format.

Such computers may be interconnected by one or more networks in anysuitable form, including a local area network or a wide area network,such as an enterprise network, and intelligent network (IN) or theInternet. Such networks may be based on any suitable technology and mayoperate according to any suitable protocol and may include wirelessnetworks, wired networks or fiber optic networks.

The various methods or processes (outlined herein may be coded assoftware that is executable on one or more processors that employ anyone of a variety of operating systems or platforms. Additionally, suchsoftware may be written using any of a number of suitable programminglanguages and/or programming or scripting tools, and also may becompiled as executable machine language code or intermediate code thatis executed on a framework or virtual machine.

In this respect, various inventive concepts may be embodied as acomputer readable storage medium (or multiple computer readable storagemedia) (e.g., a computer memory, one or more floppy discs, compactdiscs, optical discs, magnetic tapes, flash memories, circuitconfigurations in Field Programmable Gate Arrays or other semiconductordevices, or other non-transitory medium or tangible computer storagemedium) encoded with one or more programs that, when executed on one ormore computers or other processors, perform methods that implement thevarious embodiments of the invention discussed above. The computerreadable medium or media can be transportable, such that the program orprograms stored thereon can be loaded onto one or more differentcomputers or other processors to implement various aspects of thepresent invention as discussed above.

The terms “program” or “software” are used herein in a generic sense torefer to any type of computer code or set of computer-executableinstructions that can be employed to program a computer or otherprocessor to implement various aspects of embodiments as discussedabove. Additionally, it should be appreciated that according to oneaspect, one or more computer programs that when executed perform methodsof the present invention need not reside on a single computer orprocessor, but may be distributed in a modular fashion amongst a numberof different computers or processors to implement various aspects of thepresent invention.

Computer-executable instructions may be in many forms, such as programmodules, executed by one or more computers or other devices. Generally,program modules include routines, programs, objects, components, datastructures, etc. that perform particular tasks or implement particularabstract data types. Typically the functionality of the program modulesmay be combined or distributed as desired in various embodiments.

Also, data structures may be stored in computer-readable media in anysuitable form. For simplicity of illustration, data structures may beshown to have fields that are related through location in the datastructure. Such relationships may likewise be achieved by assigningstorage for the fields with locations in a computer-readable medium thatconvey relationship between the fields. However, any suitable mechanismmay be used to establish a relationship between information in fields ofa data structure, including through the use of pointers, tags or othermechanisms that establish relationship between data elements.

Also, various inventive concepts may be embodied as one or more methods,of which an example has been provided. The acts performed as part of themethod may be ordered in any suitable way. Accordingly, embodiments maybe constructed in which acts are performed in an order different thanillustrated, which may include performing some acts simultaneously, eventhough shown as sequential acts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e., “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in the United States Patent Office Manual ofPatent Examining Procedures, Section 2111.03.

The invention claimed is:
 1. A method of imaging a scene, the methodcomprising: generating a spatiotemporally varying optical intensitypattern with an array of phased modulators; illuminating a portion ofthe scene with the spatiotemporally varying optical intensity pattern soas to cause a photon to scatter or reflect off the portion of the scene;detecting the photon reflected or scattered from the portion of thescene; and estimating a distance to the portion of the scene based onthe spatiotemporally varying optical intensity pattern and a time offlight of the photon.
 2. The method of claim 1, wherein generating thespatiotemporally varying optical intensity pattern comprises modulatingcontinuous-wave beams with the array of phased modulators.
 3. The methodof claim 2, wherein illuminating the portion of the scene comprisessteering the spatiotemporally varying optical intensity pattern acrossthe portion of the scene during a time interval shorter than a dephasingtime of the continuous-wave beams.
 4. The method of claim 2, furthercomprising: periodically phase-locking the continuous-wave beamstogether.
 5. The method of claim 4, wherein periodically phase-lockingthe continuous-wave beams together comprises measuring intensity changesin the spatiotemporally varying optical intensity pattern caused bydithering relative optical phases of the continuous-wave beams.
 6. Themethod of claim 4, wherein periodically phase-locking thecontinuous-wave beams together comprises simultaneously steering thespatiotemporally varying optical intensity pattern and aligning phasesof the continuous-wave beams.
 7. The method of claim 1, whereinilluminating the portion of the scene comprises sweeping thespatiotemporally varying optical intensity pattern across the portion ofthe scene.
 8. The method of claim 1, wherein illuminating the portion ofthe scene comprises sweeping a bright fringe in the spatiotemporallyvarying optical intensity pattern across the portion of the scene withinup to about 5 ns.
 9. An apparatus for imaging a scene, the apparatuscomprising: a phased array to illuminate a portion of the scene with aspatiotemporally varying optical intensity pattern so as to cause aphoton to scatter or reflect from the portion of the scene; at least onedetector, in optical communication with the phased array, to detect thephoton scattered or reflected by the portion of the scene; and aprocessor, operably coupled to the at least one single-photon detector,to estimate a distance between the at least one detector and the portionof the scene based on a time of flight of the photon.
 10. The apparatusof claim 9, wherein the phased array is configured to sweep thespatiotemporally varying optical intensity pattern across the portion ofthe scene.
 11. The apparatus of claim 9, wherein the phased array isconfigured to sweep a bright fringe in the spatiotemporally varyingoptical intensity pattern across the portion of the scene within up toabout 5 ns.
 12. The apparatus of claim 9, wherein the phased arraycomprises an array of phased modulators to from the spatiotemporallyvarying optical intensity pattern by modulating continuous-wave beams.13. The apparatus of claim 12, wherein the phased array is configured tosteer the spatiotemporally varying optical intensity pattern across theportion of the scene during a time interval shorter than a dephasingtime of the continuous-wave beams.
 14. The apparatus of claim 12,further comprising: a light source to generate a continuous-wave beam; abeam splitter, in optical communication with the light source, to splitthe continuous-wave beam into the continuous-wave beams; and an array ofoptical amplifiers, in optical communication with the beam splitter, toamplify the continuous-wave beams.
 15. The apparatus of claim 12,further comprising: circuitry, operably coupled to the phased array, toperiodically phase-locking the continuous-wave beams together.
 16. Theapparatus of claim 15, wherein the circuitry comprises: a photodetector,in optical communication with the phased array, to measure intensitychanges in the spatiotemporally varying optical intensity pattern causedby dithering relative optical phases of the continuous-wave beams; and acontroller, operably coupled to the photodetector, to change a phasesetting of at least one phase modulator in the array of phase modulatorsbased at least in part on the intensity measured by the photodetector.17. The apparatus of claim 15, wherein the circuitry is configured tosteer the spatiotemporally varying optical intensity pattern and alignphases of the continuous-wave beams simultaneously.
 18. The apparatus ofclaim 9, wherein the at least one detector comprises an array ofsingle-photon detectors.